The preparation of polymer coatings by reacting diisocyanates and compounds with two or more reactive hydrogen atoms is generally known from DE-A 756 058. Example 10 describes the lacquering of a copper wire with a mixture of diisocyanates and a polyester which contains hydroxyl groups.
In practice, lacquers and processes, for example to produce polyurethane wire enamels, have proved to be unsuitable because it was not possible to obtain specific thicknesses of enamel coating. Capped polyisocyanates, which ought not to possess this disadvantage, were developed for this reason. (O. Bayer: Neuere Entwicklungen des Diisocyanat-Polyaddition-Verfahrens/FATIPEC 1957, p. 14 and 15 as well as Houben-Weyl, 1962, vol.14/2, p.78 and BAYER-Produkte fur die Elektroisolierung, items 1.1/2 to 1.1/3, Status 1987).
Wire lacquers made from condensation resins which contain hydroxyl groups and phenol-capped polyisocyanates are now used on a large scale for the preparation of enamelled wires which can be readily tinned. As a rule, lacquers dissolved in cresol possess solids contents of only about 25-35%. The coatings obtained using these do not possess adequate thermal properties for many applications due to the desired instability of the urethane groups.
The use of polyesterimides, polyesteramidimides, polyimides and polyamidimides as very heat-resistant electrical insulation materials for electrical conductors such as e.g. winding wires made of copper or aluminium is known. ("Moderne Electroisolierlacke--ein Fortschritt bei hochtemperaturebestandigen Kunststoffen" Kunststoffe, 61, 1971, pages 46-56, published by Carl Hanser, Munich).
Polyesterimides and polyesteramidimides are prepared e.g. by condensation of an excess of polyalcohols such as ethylene glycol, tris(hydroxyethyl)isocyanurate with dimethylterephthalate, trimellitic anhydride and diaminodiphenylmethane (DE-A 16 45 435). Cresols in combination with aromatic hydrocarbons are generally used as solvents. Cross-linking takes place with the elimination of glycols and requires a high energy input. In the case of relatively thick layers of lacquer, small bubbles or blisters may be produced by the eliminated glycols.
Higher aromatic/aliphatic polyamides are described for use as fired lacquers with particularly good thermal and chemical stability. They are dissolved in N-methylpyrrolidone and in combination with aromatic hydrocarbons and have solids contents of about 25-30% by weight at a processing viscosity of about 1000-2500 mPa.s. If it is attempted to condense these compounds to a lesser degree in order to increase the solids content then the solutions are not stable to storage. and the lacquer film properties are very poor (U.S. Pat. No. 4,501,883).
Polyimides have extraordinarily high thermal stability. They are obtained, for instance, by reacting tetracarboxylic anhydrides (pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride) and diamines (diaminodiphenylmethane, diaminodiphenylether) in strongly polar solvents (N-methylpyrrolidone, dimethylacetamide, cresol), wherein polyamido acids are produced as soluble intermediates. These are converted. into polyimides under heat treatment with the elimination of water. The solutions have low solids contents; their storability is limited because cyclisation to the imide takes place slowly even at room temperature and the water being produced leads to progressive hydrolysis of the amide bonds (see also DE-A 15 95 005 and DE-A 23 46 393).
Polyamidimides are used to insulate electrical conductors both as a base-coat and as an over-coat with good thermal, mechanical and chemical resistance. Usually, they are prepared by reacting tricarboxylic anhydrides with diisocyanates (DE-A 25 42 706, DE-A 32 49 544). In this instance it is necessary to react up to relatively high molecular weights in order to obtain the required characteristics accompanied by good storage stability for the lacquer. The lacquers prepared in this way are dissolved in N-methylpyrrolidone or cresol and have solids contents of ca. 15-35% (see also DE-A 12 56 418).
For preparation of polyamidimide lacquers with a higher solids content, DE 35 44 548 suggests mixing lower polyamidimide precursors which contain carboxyl groups with fully capped polyisocyanates. However, when using phenol-capped polyisocyanates, the storage stability of the lacquers is still inadequate. The viscosity increases greatly over a short period which impairs the ability to form a coating.
The use of alcohol-capped polyisocyanates improves the storage stability, but impairs the curing rate to an undesirable extent.
The preparation of lacquered wires usually takes place by multiple continuous applications of the wire lacquer to the copper wire using felt or nozzle wiping techniques. After each application, the lacquer is hardened at temperatures between ca. 300.degree. and 700.degree. C. The solvent and capping agent are eliminated during the cross-linking process and escape from the lacquer film. They are generally catalytically incinerated and emitted to the environment in the form of CO.sub.2. To eliminate the capping agent, additional energy has to be supplied during the hardening process.
When using capped polyisocyanates, several reactions take place at the same time during heat-curing. On the one hand, the solvents evaporate, while at the same time a relatively large decrease in the viscosity of the binder occurs on raising the temperature. In parallel with this, cross-linking takes place with elimination of the capping agent. The process is extraordinarily complex and it is difficult to achieve higher rates of processing at increased solids contents without impairing the surface of the lacquer.
The elevated temperature of between ca. 300.degree. and 700.degree. C. required to form the film may even lead to pyrolytic degradation reactions during the hardening process.